How the tapir got his spots III

August 13, 2009 • 12:49 pm

by Greg Mayer

The two great classes of phenomena that Darwin set out to explain were those of adaptation– the fit between an organism’s features (structure, behavior, etc.) and its conditions of existence; and unity of type — the similarities of basic structure among organisms in diverse conditions of existence (e.g., the one bone-two bones-many bones pattern of tetrapod forelimbs, whether they be burrowers, swimmers, climbers, runners, etc.). The unified explanation that Darwin provided for these phenomena was descent with modification: the similarities were due to inheritance from a common ancestor (i.e. descent), while adaptation arose from the process of modification (i.e. natural selection).

The methods of studying adaptation are thus crucial for biology.  How can we tell what (if anything) the spots of the baby tapir are adaptive for?

There are three basic ways of studying adaptation, in the sense of determining what a trait is an adaptation to. The first is engineering: does the feature conform to what we would expect if it is performing some adaptive function?  Study of hydrodynamics enables us to understand the shapes of the bodies, flippers, and fins in fish, dolphins, icthyosaurs, etc. as adaptations to movement within a fluid environment.  The dorsal fin of an ichthyosaur, for example, stabilizes the reptile in its forward movement through water, preventing unwanted roll (for recent discussions of ichthyosaur aquatic adaptations, see here, here, and here). For another example of the engineering approach, see Richard Dawkins’ delightful account of bat sonar in chap. 2 of The Blind Watchmaker.

Second, there is the method of correlation (also called the comparative method): does the feature evolve repeatedly in particular environmental circumstances? Thus even if we were wholly ignorant of hydrodynamics, the repeated evolution of dorsal fins in aquatic fish, reptiles, and mammals provides evidence that dorsal fins are adaptations to an aquatic existence.


Third, we can study the effects on survival and reproduction of variations in the trait of interest.  This can be done either by altering the features of the character experimentally (as in this neat experiment on sexual selection in widowbirds) or by studying naturally occurring variants (as was done with peppered moths by  H.B.D. Kettlewell).

The evidence for the adaptiveness of spotting/striping in mammals is primarily of the first sort (Hugh B. Cott, in his classic Adaptive Coloration in Animals, has a lot about optical principles, and what makes things hard to see), the second sort (pacas, bongos, deer, tapirs all have spots and/or stripes [and note that pacas are rodents, and that tapirs, which are perissodactyls, are not at all closely related to the artiodactyl deer and bongo, so it would be hard to argue it’s a retained ancestral feature]), and very little of the third sort– no one’s painted baby tapirs’ spots over to see what happens to them (at least as far as I know). I’ll touch on all three sorts as they relate to tapirs in later posts.

(For other examples of camouflage, see Matthew Cobb’s earlier post on the subject.)